For ready-prepared meals, there are currently double-figure growth rates in Europe. The ready-prepared meals are transferred to trays after their preparation (cf. FIG. 1). A film which is hot-sealed to the edge of the tray seals the packaging and protects the ready-prepared meal from external influences. The ready-prepared meals are suitable, for example, for heating in a microwave, for heating in a conventional oven or for heating in a microwave and in a conventional oven. In the latter case, the ready-prepared meal and the packaging have to be “dual ovenable” (=suitable for microwave and conventional ovens). As a consequence of the temperatures existing in the conventional oven (up to 220° C.), particularly high requirements are made on the packaging materials (tray and lid film).
Both for the tray and for the lid film, only selected materials can be considered for dual ovenable applications. Typical materials for the trays are in this case CPET, aluminum, cardboard coated with PET or with PET film. In the case of trays made of CPET (cf. FIG. 1), the thick crystalline CPET layer which is usually pigmented, i.e. filled with particles, provides the stability of the tray, even at comparatively high temperatures in the conventional oven. Compared to other trays, for example made of amorphous APET, CPET trays have poorer adhesion properties.
In dual ovenable applications, the material used for the lid film is generally PET which is sufficiently dimensionally stable and solid even at 220° C. Materials such as PP or PE are ruled out from the outset because of their low melting points. The requirements on the lid film are best fulfilled by biaxially oriented polyester film.
When preparing the ready-prepared meal in the oven, the polyester film is removed by hand from the tray shortly before heating or shortly after heating. When this is done, the polyester film must on no account start to tear, start and continue to tear or tear off. The removal of the film from the tray without the film starting or continuing to tear or tearing off is also referred to in the foods industry as peeling. For this application, the polyester film therefore has to be not only heatsealable, but in particular also peelable. For a given material and given overall thickness of the film, the peelability of the film is determined mainly by the properties of the surface layer of the film which is sealed to the tray.
The peelability of films can be determined relatively simply in the laboratory using a tensile strain tester (for example Zwick) (cf. FIG. 2). For this test, two strips of breadth 15 mm and length approx. 50 mm are first cut out of the polyester film and the tray and sealed together. The sealed strips are, as shown in FIG. 2, clamped into the clips of the tester. The “angle” between the film clamped in the upper clip and the tray strip is 180°. In this test, the clips of the tester are moved apart at a speed of 200 mm/min, and in the most favorable case, the film is fully removed from the tray (based on ASTM-D 3330).
In this test, a distinction is to be drawn between essentially two different mechanisms.
In the first case, the tensile force rises rapidly in the course of the pulling procedure up to a maximum (cf. FIG. 3a) and then falls directly back to zero. When the maximum force is attained, the film starts to tear, or before delamination from the tray, tears off, resulting in the force falling immediately back to zero. The film is in this case not peelable, since it is destroyed. The behavior of the film can rather be described as a kind of “welding” to the tray. The destruction of the film on removal from the tray is undesired, because this complicates the easy opening of the packaging without tools such as scissors or knives.
In contrast, a peelable film is obtained when the tensile force or the peeling force rises up to a certain value (i.e. up to a certain plateau) and then remains approximately constant over the distance over which the two strips are sealed together (cf. FIG. 3b). In this case, the film does not start to tear, but rather can be peeled off as desired from the tray with a low force input.
The size of the peeling force is determined primarily by the polymers used in the sealing layer (A) (cf. FIG. 4, polymer 1 and polymer 2). In addition, the size of the peeling force is dependent in particular on the heatsealing temperature employed. The peeling force generally rises with the heatsealing temperature. With increasing heatsealing temperature, the risk increases that the sealing layer might lose its peelability. In other words, a film which is peelable when a low heatsealing temperature is employed loses this property when a sufficiently high heatsealing temperature is employed. This behavior is to be expected in particular in the case of polymers which exhibit the characteristics shown in FIG. 4 for polymer 1. This behavior which tends to generally occur but is rather unfavorable for the application has to be taken into account when designing the sealing layer. It has to be possible to heatseal the film in a sufficiently large temperature range without the desired peelability being lost (cf. polymer 2 in FIG. 4). In practice, this temperature range is generally from 150 to 220° C., preferably from 150 to 200° C. and more preferably from 150 to 190° C.
The heatsealable and peelable layer is applied to the polyester film in accordance with the prior art, generally by means of offline methods (i.e. in an additional process step following the film production). This method initially produces a “standard polyester film” by a customary process. The polyester film produced in this way is then coated in a further processing step in a coating unit offline with a heatsealable and peelable layer. In this process, the heatsealable and peelable polymer is initially dissolved in an organic solvent. The final solution is then applied to the film by a suitable application process (knifecoater, patterned roller, die). In a downstream drying oven, the solvent is evaporated and the peelable polymer remains on the film as a solid layer.
Such an offline application of the sealing layer is comparatively expensive for several reasons. First, the film has to be coated in a separate step in a special apparatus. Second, the evaporated solvent has to be condensed again and recycled, in order thus to minimize pollution of the environment via the waste air. Third, complicated control is required to ensure that the residual solvent content in the coating is very low.
Moreover, in an economic process, the solvent can never be completely removed from the coating during the drying, in particular because the drying procedure cannot be of unlimited duration. Traces of the solvent remaining in the coating subsequently migrate via the film disposed on the tray into the foods where they can distort the taste or even damage the health of the consumer.
Various peelable, heatsealable polyester films which have been produced offline are offered on the market. The polyester films differ in the structure and in the composition of the top layer (A). Depending on their (peeling) properties, they have different applications. It is customary, for example, to divide the films from the application viewpoint into films having easy peelability (easy peel), having medium peelability (medium peel) and having strong, robust peelability (strong peel). The essential quantifiable distinguishing feature between these films is the size of the particular peeling force according to FIG. 3b. A division is carried out at this point as follows:
Easy peelabilityPeeling force in the range of(easy peel)from about 1 to 4 N per 15 mmof strip breadthMedium peelabilityPeeling force in the range(medium peel)from about 3 to 8 N per 15 mmof strip breadthHigh, robust peelabilityPeeling force in the range of(strong peel)more than 5 N per 15 mm ofstrip breadth
Some sealable PET films are already known.
EP-A-0 035 835 describes a coextruded sealable polyester film to which particles whose average particle size exceeds the layer thickness of the sealing layer are added in the sealing layer to improve the winding and processing performance. The polymer of the sealing film layer is substantially a polyester copolymer which is based on aromatic dicarboxylic acids and also aliphatic diols. The particulate additives form surface elevations which prevent undesired blocking and adhesion of the film to rolls or guides. The selection of particles having a diameter greater than the sealing layer worsens the sealing performance of the film. No information is given in the document on the sealing temperature range of the film. The seal seam strength is measured at 140° C. and is in the range from 63 to 120 N/m (corresponding to from 0.97 to 1.8 N/15 mm of film breadth). There are no indications in the document concerning the peeling performance of the film with respect to trays made of CPET.
EP-A-0 379 190 describes a coextruded, biaxially oriented polyester film which comprises a carrier film layer made of polyester and at least one sealing film layer made of a polyester composition. The sealing film layer may comprise aliphatic and aromatic dicarboxylic acids and also aliphatic diols. The polymer for the sealing film layer comprises two different polyesters A and B, of which at least one (polyester B) contains aliphatic dicarboxylic acids and/or aliphatic diols. The sealing energy which is measured between two sealing film layers facing each other and joined together (=fin sealing) is more than 400 gforce·cm/15 mm (more than 4 N·cm/15 mm), and the sealing film layer may comprise inorganic and/or organic fine particles which are insoluble in the polyester, in which case the fine particles are present in an amount of from 0.1 to 5% by weight, based on the total weight of the sealing film layer. In the examples of EP-A-0 379 190, organic particles, when they are used at all, are used in maximum amounts of 0.3% by weight. Although the film features good peeling properties (having plateau character in the peeling diagram [see above]) with respect to itself (i.e. sealing film layer with respect to sealing film layer), there is no information about the peeling performance with respect to trays made of CPET. In particular, the film of this invention is in need of improvement in its producibility and its processibility (the raw materials tend to adhere).
WO A-96/19333 describes a process for producing peelable films, in which the heatsealable, peelable layer is applied inline to the polyester film. In the process, comparatively small amounts of organic solvents are used. The heatsealable, peelable layer comprises a copolyester for which a) from 40 to 90 mol % of an aromatic dicarboxylic acid, b) from 10 to 60 mol % of an aliphatic dicarboxylic acid, c) from 0.1 to 10 mol % of a dicarboxylic acid containing a free acid group or a salt thereof, d) from 40 to 90 mol % of a glycol containing from 2 to 12 carbon atoms and e) from 10 to 60 mol % of a polyalkyldiol for forming the copolyester were used. The coating is applied to the film from an aqueous dispersion or a solution which contains up to 10% by weight of organic solvent. The process is restricted with regard to the polymers which can be used and the layer thicknesses which can be achieved for the heatsealable, peelable layer. The maximum achievable layer thickness is specified as 0.5 μm. The maximum seal seam strength is low, and is from 500 to 600 g/25 mm2, or [(from 500 to 600)/170] N/15 mm of film breadth.
WO 02/059186 A1 describes a process for producing peelable films, in which the heatsealable, peelable layer is likewise applied inline to the polyester film. In this case, melt-coating is employed, and it is preferably the longitudinally stretched film which is coated with the heatsealable, peelable polymer. The heatsealable, peelable polymer contains polyesters based on aromatic and aliphatic acids, and also based on aliphatic diols. The copolymers disclosed in the examples have glass transition temperatures of below −10° C.; such copolyesters are too soft, which is why they cannot be oriented in customary roll stretching methods (adhesion to the rolls). The thickness of the heatsealable, peelable layer is less than 8 μm. In WO 02/059186 A1, the melt-coating known per se is delimited from the extrusion coating known per se technically and by the viscosity of the melt. A disadvantage of the process is that only comparatively fluid polymers (max. 50 Pa*sec) having a low molecular weight can be used. This results in disadvantageous peeling properties of the film. Moreover, the coating rate in this process is limited, which makes the production process uneconomic. With regard to quality, faults are observed in the optical properties of the film which are visible, for example, as coating streaks. In this process, it is also difficult to obtain a uniform thickness of the sealing layer over the web breadth of the film, which in turn leads to nonuniform peeling characteristics.